This invention relates to digital loop carrier transmission systems.
In present digital loop carrier transmission systems, such as the Subscriber Loop Carrier (SLC.RTM.) Series 5 system, data communications take place between a switching entity at the central office (CO) and a remote terminal (RT) in the field. A "switching entity" is defined as a part of a switch that includes call processing capability plus a portion of the switching network. Each switching entity has a traffic-handling capacity which can be reached when the number and duration of calls to and from the remote terminal attain a certain level. It is often desirable, therefore, to move customer lines to different switching entities in order to balance the load on the different parts of the central office switch. In typical prior art systems, all customer lines in a remote terminal would be moved to a new switching entity. This meant that if a transfer of RT/CO feeder facilities to the new switching entity was performed halfway through the CO data transfer process, the average customer would be without telephone service for the time it takes to transfer one-quarter of all customers' data to the new switching entity. Alternatively, such load balancing problems could be handled by switch hardware and software modifications, but this approach was expensive.
Digital loop carrier systems presently employed in the United States conform to one of two standards for interfacing with the digital switches. The older standard, known generally in the industry as TR8, allows a remote terminal with no more than 96 customer lines. The newer standard, known as TR303, is compatible with remote terminals ranging from 48-2048 lines. In the case of TR8 systems, where it is desired to increase the number of lines to a remote terminal to greater than the allowed 96 lines, it has been proposed that a plurality of "virtual" remote terminals be set up at each remote terminal site. That is, each switching entity could be coupled to one of a plurality of virtual remote terminals set up within one physical remote terminal, and the channel units at that terminal could be mapped to any of the virtual terminals through a time slot interchanger. The result is that each remote terminal, while physically a single entity, would function in relationship to the central office as if it were a plurality of separate terminals. Since each virtual terminal in TR 8 is limited to 96 lines, however, the load balancing issue has not been addressed.
It is, therefore, an object of the invention to provide a mechanism for transferring phone lines among central office switch components with reduced per customer outage time.